In the present day, remote transponders are used in a broad variety of fields and applications of technology, preferably for example in the context of non-contact or contact-free identification systems, especially using a wireless data transmission via electromagnetic waves in the radio frequency (RF) range between the transponder and a base station. Such radio frequency identification (RFID) is suitable and applicable in all situations in which an automatic identification, characterization, recognition, interrogation, storage or stocking, monitoring, control, or transport of various objects is to be achieved. For example, using such RFID transponders, it is possible to individually mark and identify storage and transport containers, machines, vehicles, products, parts of products, persons, etc.
For individually identifying the object to which the RFID transponder, or so-called “tag” or “label”, is attached, the transponder comprises or is associated with a data carrier that stores, contains or embodies the identifying information that is to be interrogated. Thus, the RFID transponder, through its wireless communication with a base station, or so-called “reader” or “interrogator”, makes it possible to identify the object that has been tagged or labeled with the transponder, without contact, without a visual sight connection, and independently from the base station “reader”. In this regard, the transponder may be located directly on the pertinent object, or in or on an associated transport device such as a container, a pallet, packaging, or the like.
Since even adverse environmental influences or weather conditions do not create any obstacle or problem for the transmission of electromagnetic waves, the wireless communication of data between the transponder and the base station can be carried out reliably, securely, and error-free over relatively large spacing distances of up to 2 m or more.
Another important application of such RFID systems is in the field of motor vehicle technology, and particularly in anti-theft systems and blocking systems to prevent the driving-away of the vehicle, in which the transponder is installed in the motor vehicle ignition key, for example.
As mentioned above, an RFID system generally includes two main components, namely the transponder (tag or label) and the base station (reader or interrogator). The transponder is typically a passive device that does not include its own power supply, but instead extracts its required operating power from the electromagnetic field emitted by the base station and received by the transponder. The transponder is portable and remote, and is arranged on the object to be identified. On the other hand, the base station or reader may be stationary or portable and mobile. The transponder, as the core component of such a system, comprises an integrated circuit (IC) as a data carrier as well as a transmitting and receiving arrangement including a radio antenna. The base station or reader comprises a control unit, a frequency module, and at least one transmitting and receiving arrangement including a radio antenna.
The transponder further provides the possibility of storing data, which may be changed, updated, or replaced as needed simply by reprogramming the IC of the transponder, for example in comparison to the fixed information permanently stored in bar code labels or the like.
The data exchange or communication between the transponder and the base station is carried out via an electromagnetic field in any one or more frequencies in various frequency ranges, preferably in the RF range as mentioned above, and especially in the ultrahigh frequency (UHF) or microwave (MW) ranges.
In many applications, a passive RFID system will include a plurality of transponders that are located in a single common RF field, for example emitted by a single base station or reader, whereby all of these transponders are to be read-out by the single base station. In this context, it is typical to employ so-called anti-collision procedures so as to serially interrogate and read-out the several transponders one at a time, while avoiding collisions, interference and crosstalk caused by several transponders communicating at once. After reading-out a particular data set, for example an identification number (ID) out of the integrated circuit of a particular transponder, this transponder is then switched to an inactive mode, so that it becomes silent or mute and no longer participates in a communication with the base station or reader. Thus, thereafter further transponders can be read-out in succession with as little interference as possible. Such general anti-collision procedures are described, for example, in the Publication “RFID Handbuch” (“RFID Handbook”) by Finkenzeller, published by Hanser Publishers of Munich Germany.
In order to ensure the proper intended behavior of each transponder according to the anti-collision procedure or protocol, each transponder must maintain or hold its appropriate control condition or status throughout the entire procedure. Most importantly, this requires at least that a particular transponder, after the successful reading-out of its data set, no longer participates in and does not re-join in the communication with the base station. This aspect also becomes problematic or difficult because the passive transponders must obtain all their operating energy requirements by extracting the energy from the RF field emitted by the base station. In certain situations, so-called field gaps arise in the RF field, which result in an inadequate energy supply for any transponder located in such a field gap.
This aspect becomes especially critical when the carrier frequencies for the communication between the transponder and the base station are in the UHF or MW ranges, and especially if the positions of the transponders in space vary or move relative to the base station. Namely, especially in the UHF and MW ranges, such spatial areas or field gaps in which the energy supply for the transponder from the carrier signal is no longer adequate arise due to the superimposing of reflections of the field, and the like.
Thus, when a particular transponder is located in such a spatial area or field gap, it is necessary that the transponder supplies its ongoing operating energy requirement from an on-board energy storage device located on the transponder, typically comprising a capacitor. Thus, energy stored in the capacitor is used to “bridge over” the energy requirements of the transponder for temporary periods when it is located in such a field gap with an inadequate energy supply from the emitted RF field. Since the time periods for bridging over such field gaps can become rather long, and are typically in the range of several seconds, the on-board storage capacitor (or capacitors) must have a relatively large storage capacity, e.g. on the order of microfarads. Thus, the energy storage arrangement consumes a relatively large surface area on the integrated circuit of the transponder, so that such attempted solutions of the field gap problem have been structurally, practically, and economically unacceptable. For this reason, there have been prior efforts to ensure that such field gaps resulting in a temporary inadequate power supply for the transponder do not have negative influences on the anti-collision procedures.
U.S. Pat. No. 5,963,144 (Kruest) discloses a transponder, a circuit arrangement and a method for controlling such a transponder and circuit arrangement in the general field as discussed above. Particularly, after the transponder is recognized and registered by the base station, the transponder is completely switched off for a time duration t, whereby t amounts to approximately two seconds. After expiration of this time t, the pertinent transponder will automatically rejoin and latch into the ongoing communication with the base station, independently of whether or not the anti-collision procedure has already been completed. According to the cited US patent, the respective transponder is switched off and removed from the communication by adjusting the input impedance of the transponder. This is disadvantageous, however, because the pertinent transponder can thus not be interrogated or communicated with for other reasons, i.e. for any reason or in any manner, during the time period in which it is switched to the inactive mode. Moreover, it becomes problematic that undefined control conditions or states of the transponder can arise as a result of power supply gaps, because there is no way for the transponder to hold or “remember” the particular proper existing control condition throughout such a power supply gap. Thus, after expiration of the off-line or inactive time t, such undefined control conditions could possibly be transmitted to the entire system.
While the basic state of the art underlying the present invention, as generally discussed above, has related to remote or autonomous passive transponders, the same considerations also apply to so-called remote sensors. A remote sensor can be considered as a special type of remote transponder devices that further includes a sensor for sensing the local existing data that are to be transmitted back to the base station. Namely, while the transponders discussed above communicate data that has been stored on-board the transponder (i.e. a stored data or identification transponder), a remote sensor is a transponder further including a sensor, for example for sensing temperature, pressure, motion, gas composition, particles, light intensity, or the like, so that the remote sensor can then wirelessly communicate the sensed data back to the base station. Just like any remote transponder, the remote sensor receives its operating energy as well as control instructions (e.g. sensing or measuring instructions) from the base station.
Throughout this specification, the terms “transponder” or “remote transponder” should be understood to cover both an identification transponder or stored data transponder as well as a remote sensor transponder, unless the specific context indicates that one or the other type of transponder is specifically being addressed.